cmd/go: provide a more helpful suggestion for "go vet -?"

For the command

	go vet -?

the output was,

	usage: go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
	Run 'go help vet' for details.
	Run 'go tool vet -help' for the vet tool's flags.

but "go help vet" is perfunctory at best. (That's another issue I'm
working on—see https://go-review.googlesource.com/c/tools/+/291909—
but vendoring is required to sort that out.) Add another line and rewrite
a bit to make it actually helpful:

	usage: go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
	Run 'go help vet' for details.
	Run 'go tool vet help' for a full list of flags and analyzers.
	Run 'go tool vet -help' for an overview.

Change-Id: I9d8580f0573321a57d55875ac3185988ce3eaf64
Reviewed-on: https://go-review.googlesource.com/c/go/+/291929
Trust: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/vet/vetflag.go

@@ -184,7 +184,8 @@ func exitWithUsage() {
 	if vetTool != "" {
 		cmd = vetTool
 	}
-	fmt.Fprintf(os.Stderr, "Run '%s -help' for the vet tool's flags.\n", cmd)
+	fmt.Fprintf(os.Stderr, "Run '%s help' for a full list of flags and analyzers.\n", cmd)
+	fmt.Fprintf(os.Stderr, "Run '%s -help' for an overview.\n", cmd)
 
 	base.SetExitStatus(2)
 	base.Exit()

src/cmd/go/testdata/script/help.txt

@@ -34,9 +34,10 @@ stderr 'Run ''go help mod'' for usage.'
 # Earlier versions of Go printed the same as 'go -h' here.
 # Also make sure we print the short help line.
 ! go vet -h
-stderr 'usage: go vet'
-stderr 'Run ''go help vet'' for details'
-stderr 'Run ''go tool vet -help'' for the vet tool''s flags'
+stderr 'usage: go vet .*'
+stderr 'Run ''go help vet'' for details.'
+stderr 'Run ''go tool vet help'' for a full list of flags and analyzers.'
+stderr 'Run ''go tool vet -help'' for an overview.'
 
 # Earlier versions of Go printed a large document here, instead of these two
 # lines.